Run Time Login

Hi Everyone,
I am trying to switch login name inside the NT.For example.Lets say I have 2 user A/C in NT.names UserA,UserB.Both user have different previlaged.UserA can Access the files created by UserB,UserA can not delete the files created by UserB but UserB can delete files created By UserA..So what I wanted to do Login as a UserA in the system (Windows NT).And at the run Time i.e inside my Application I want to switch login from UserA to UserB.Will it be possible?If yes the the How?
can any body give any Idea in this.
Thanx in Advance

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